The present invention pertains generally to medical devices and their methods of manufacture, and more particularly to tubular, polytetrafluoroethylene (PTFE) grafts having integral, radially expandable stents, for implantation in cavities or passageways (e.g., ducts or blood vessels) of the body.
A. Stents
The prior art has included a number of radially expandable stents which may be initially deployed in a radially collapsed state suitable for transluminal insertion via a delivery catheter, and subsequently transitioned to a radially expanded state whereby the stent will contact and engage the surrounding wall or the anatomical duct or body cavity within which the stent has been positioned. Such stents have been used to support and maintain the patency of blood vessel lumens (e.g., as an adjuvant to balloon angioplasty) and to structurally support and/or anchor other apparatus, such as tubular endovascular grafts, at desired locations within a body cavity or passageway (e.g., to anchor a tubular endovascular graft within a blood vessel such that the graft forms an internal conduit through an aneurysm or site of traumatic injury to the blood vessel wall).
Many stents of the prior art have been formed of individual member(s) such as a wire, plastic, metal strips, or mesh which has been bent, woven, interlaced or otherwise fabricated into a generally cylindrical configuration. These stents of the prior art have generally been classified into two major categoriesxe2x80x94a) xe2x80x9cself-expandingxe2x80x9d stents, and b) xe2x80x9cpressure expandablexe2x80x9d stents.
i) Self-expanding Stents
Self-expanding stents are typically formed of spring metal, shape memory alloy, or other material which is resiliently biased toward its"" fully radially expanded Configuration or otherwise capable of self-expanding to its"" fully radially expanded configuration without the need for the exertion of outwardly directed radial force upon the stent by some extraneous expansion apparatus (e.g., a balloon or mechanical expander tool). These self-expanding stents may be initially radially compressed and loaded into a small diameter delivery catheter or alternatively mounted upon the outer surface of a delivery catheter equipped with some means for restraining or maintaining the stent in its"" radially compressed state. Thereafter, the delivery catheter is inserted into the body and is advanced to a position where the stent is located at or near the site at which it is to be implanted. Thereafter, the stent is expelled out of (or released from) the delivery catheter and allowed to self-expand to it""s full radial diameter. Such expansion of the stent causes the stent to frictionally engage the surrounding wall of the body cavity or passageway within which the stent has been positioned. The delivery catheter is then extracted, leaving the self-expanded stent at its"" intended site of implantation. Some examples of self-expanding stents of the prior art include those described in U.S. Pat. No. 4,655,771 (Wallsten et al.); U.S. Pat. No. 4,954,126 (Wallsten): U.S. Pat. No. 5, 061, 275 (Wallsten et al.); U.S. Pat. No. 4,580,568 (Gianturco); U.S. Pat. No. 4,830,003 (Wolf et al.); U.S. Pat. No. 5,035,706 (Gianturco et al.) and U.S. Pat. No. 5,330,400 (Song).
ii) Pressure-Expandable Stents
The pressure-expandable stents of the prior art are typically formed of metal wire, metal strips, or other malleable or plastically deformable material, fabricated into a generally cylindrical configuration. The pressure-expandable stent is initially disposed in a collapsed configuration having a diameter which is smaller than the desired final diameter of the stent, when implanted in the blood vessel. The collapsed stent is then loaded into or mounted upon a small diameter delivery catheter. The delivery catheter is then advanced to its desired location within the vasculature, and a balloon or other stent-expansion apparatus (which may be formed integrally of or incorporated into the delivery catheter) is utilized to exert outward radial force on the stent, thereby radially expanding and plastically deforming the stent to it""s intended operative diameter whereby the stent frictionally engages the surrounding blood vessel wall. The material of the stent undergoes plastic deformation during the pressure-expansion process. Such plastic deformation of the stent material causes the stent to remain in its radially expanded operative configuration. The balloon or other expansion apparatus is then deflated/collapsed and is withdrawn from the body separately from, or as part of, the delivery catheter, leaving the pressure-expanded stent at it""s intended site of implantation.
Some examples of pressure-expandable stents of the prior art include those described in U.S. Pat. No. 5,135,536 (Hillstead); U.S. Pat. No. 5,161,547 (Tower); U.S. Pat. No. 5,292,331 (Boneau); U.S. Pat. No. 5,304,200 (Spaulding) and U.S. Pat. No. 4,733,665 (Palmaz).
B. PTFE Vascular Grafts
Fluoropolymers, such as polytetrafluoroethylene, have been heretofore used for the manufacture of various types of prosthetic vascular grafts. These vascular grafts are typically of tubular configuration so as to be useable to replace an excised segment of blood vessel.
The tubular PTFE vascular grafts of the prior art have traditionally been implanted, by open surgical techniques, whereby a diseased or damaged segment of blood vessel is surgically excised and removed, and the tubular bioprosthetic graft is then anastomosed into the host blood vessel as a replacement for the previously removed segment thereof. Alternatively, such tubular prosthetic vascular grafts have also been used as bypass grafts wherein opposite ends of the graft are sutured to a host blood vessel so as to form a bypass conduit around a diseased, injured or occluded segment of the host vessel.
In general, many tubular prosthetic vascular grafts of the prior art have been formed of extruded, porous PTFE tubes. In some of the tubular grafts of the prior art a PTFE tape is wrapped about and laminated to the outer surface of a tubular base graft to provide reinforcement and additional burst strength. Also, some of the prior tubular prosthetic vascular grafts have included external support member(s), such as a PTFE beading, bonded or laminated to the outer surface of the tubular graft to prevent the graft from becoming compressed or kinked during implantation. These externally supported tubular vascular grafts have proven to be particularly useful for replacing segments of blood vessel which pass through, or over, joints or other regions of the body which undergo frequent articulation or movement.
One commercially available, externally-supported, tubular vascular graft is formed of a PTFE tube having a PTFE filament helically wrapped around, and bonded to, the outer surface of the PTFE tube. (IMPRA Flex(trademark) Graft, IMPRA, Inc., Tempe, Ariz.)
One other commercially available, externally-supported, tubular vascular graft comprises a regular walled, PTFE tube which has PTFE reinforcement tape helically wrapped around, and bonded to,the outer surface of the PTFE tube and individual rings of Fluorinated Ethylene Propylene (FEP) rings disposed around, and bonded to, the outer surface of the reinforcement tape. (FEP ringed ePTFE vascular graft, W. L. Gore and Associates, Inc., Flagstaff, Ariz.)
C. Stented Grafts
The prior art has also included a number of xe2x80x9cstented graftsxe2x80x9d. These stented grafts typically comprise a self-expanding or pressure-expandable stent which is affixed to or formed within a pliable tubular graft. Because of their radial compressibility/expandability, these stented grafts are particularly useable in applications wherein it is desired to insert the graft into an anatomical passageway (e.g., blood vessel) while the graft is in a radially compact state, and to subsequently expand and anchor the graft to the surrounding wall of the anatomical passageway.
More recently, methods have been developed for introducing and implanting tubular prosthetic vascular grafts within the lumen of a blood vessel, by percutaneous or minimal incision means. Such endovascular implantation initially involves translumenal delivery of the graft, in a compacted state, by way of a catheter or other transluminally advancable delivery apparatus. Thereafter, the graft is radially expanded and anchored to the surrounding blood vessel wall, thereby holding the graft at its intended site of implantation within the host blood vessel. An affixation apparatus such as a stent, is typically utilized to anchor at least the opposite ends of the tubular graft to the surrounding blood vessel wall. One particular application for endovascular grafts of this type is in the treatment of vascular aneurysms without requiring open surgical access and resection of the aneurysmic blood vessel. Also, such stented grafts may also be useable to treat occlusive vascular diseasexe2x80x94especially in cases where the stented graft is constructed in such a manner that the tubular graft material forms a complete barrier between the stent and the blood which is flowing through the blood vessel. In this manner the tubular graft material may serve as a smooth, biologically compatible, inner xe2x80x9ccoveringxe2x80x9d for the stent, thereby preventing a) turbulent blood-flow as the blood flows over the wire members or other structural material of which the stent is formed, b) immunologic reaction to the metal or other material of which the stent is formed, and c) a barrier to separate a diseased or damaged segment of blood vessel from the blood-flow passing therethrough. Such prevention of turbulent blood-flow and/or immunologic reaction to the stent material is believed to be desirable as both of these phenomena are believed to be associated with thrombus formation and/or restenosis of the blood vessel.
Other uses for stented grafts may include restoring patency to, or re-canalizing, other anatomical passageways such as ducts of the biliary tract, digestive tract and/or genito-urinary tract.
Many of the stented grafts known in the prior art have utilized woven or knitted material, such as polyester fiber, as the graft material.
There exists a need for the development of a radially expandable, stented graft formed of a continuous, tubular ePTFE tube because the inherent properties of PTFE may offer various clinical advantages over the woven polyester and other graft materials which have been previously used in stented grafts of the prior art.
The present invention is directed to stented, tubular, PTFE grafts and their methods of manufacture. In general, the present invention may exist in any of three (3) separate embodiments, depending upon whether the stent component of the graft is formed integrally (i.e., within) the tubular PTFE graft, externally (i.e., on the outer surface of) the tubular PTFE graft, or internally (i.e., on the inner lumenal surface) of the PTFE tubular graft. Each of these three separate embodiments of the invention may be self expanding (i.e., incorporating a self-expanding stent) or pressure-expandable (i.e., incorporating a pressure-expandable stent.
In accordance with a first embodiment of the invention, there is provided an integrally stented PTFE graft which comprises a tubular PTFE base graft preferably of a density less than 1.6 g/cc, a radially expandable stent surrounding the outer surface of the tubular base graft, and an outer PTFE layer having a density of less than 1.6 g/cc. The tubular outer layer is fused to the tubular base graft through lateral openings or perforations formed in the stent. A polymer coating, such as a PTFE coating, may be disposed on the stent to further facilitate fusing or boding of the stent to the base tube and/or outer tubular layer.
In accordance with a second embodiment of the invention, there is provided an externally stented, tubular PTFE graft which comprises a radially compressible/expandable stent having a ePTFE tube of less than 1.6 g/cc density coaxially disposed within the stent, with the outer surface of the tubular ePTFE graft being fused or attached to the stent. A polymer coating, such as PTFE or any other plastic which may be fused or adhered to PTFE, may be applied to or formed on the stent to facilitate the desired fusion or attachment of the tube graft to the stent, and/or to improve the biocompatibility of the stent.
In accordance with a third embodiment of the invention, there is provided an internally stented, tubular PTFE graft comprising a tubular outer layer formed of ePTFE having a density of less than 1.6 g/cc, and a radially expandable stent. The stent is coaxially disposed within the lumen of the tubular outer layer, and fused or attached thereto. The stent may be covered with a polymer coating, such as PTFE or other biocompatable plastic capable of adhering or fusing to PTFE, to facilitate the desired fusion or attachment of the stent to the outer tubular layer, and/or to improve the biocompatability of the stent. Additionally or alternatively, PTFE particles may be disposed between the tubular outer layer and the tubular base graft to facilitate adhering or fusing of these two layers to one another, and/or to the stent. Such PTFE particles may be disposed between the inner base graft and outer tubular layer by applying or depositing PTFE liquid dispersion therebetween, or by depositing dry PTFE resin powder therebetween.
Any of above-summarized three (3) separate embodiments of the invention may be manufactured by a method which comprises the steps of: a) initially positioning a generally cylindrical stent of either the self-expanding or pressure-expandable variety in contacting coaxial relation with the tubular ePTFE base graft and/or the tubular ePTFE outer layer, upon a cylindrical mandrel or other suitable support surface, and b) subsequently fusing the fuse (i.e., heating to a lamination temperature) assembled components (i.e., the stent in combination with the inner base graft and/or outer tubular layer) of the stented graft into a unitary stented graft structure. In integrally stented embodiments where both the tubular ePTFE base graft and the tubular ePTFE outer layer are present, such heating will. additionally cause the tubular outer layer to fuse to the inner tubular base graft, through lateral openings or perforations which exist in the stent. The stent may be surface treated, abraded, or coated with a plastic capable of adhering or fusing to ePTFE to facilitate attachment of the stent to the adjacent outer layer and/or inner base graft upon subsequent heating, application of solvent or other suitable adhesion promoting technique. In instances where a plastic coating is formed on the stent, such coating may be in the nature of a tube or film which is applied to the stent prior to assembly and mounting of the stented graft components on the mandrel or other support surface. Also, in embodiments where both the outer tubular layer and tubular base graft are used, aqueous PTFE dispersion, powdered PTFE resin or other flowable plastic material, may be deposited between the outer tubular layer and inner tubular base graft at the time of assembly (prior to heating) to further facilitate fusion of the outer tubular layer and/or inner tubular base graft to the stent and/or to one another.
By the above-described materials and methods of construction, the stented PTFE grafts of the present invention are capable of radially expanding and contracting without excessive puckering, wrinkling or invagination of the PTFE graft material. Furthermore, in embodiments wherein the stent is constructed of individual members which move or reposition relative to one another during respective expansion and contraction of the stented graft, the manufacturing methods and materials of the present invention render the PTFE sufficiently strong and sufficiently firmly laminated or fused so as to permit such relative movement of the individual members of the stent without tearing or rupturing of the tubular PTFE graft.